The nervous system, which is mainly built up by neurons and glial cells, is our most complex organ system. Neurons are the cells responsible for transmitting, processing, and storing information. During embryonic development, neurons extend long processes (axons) that follow well-defined, intricate pathways and make predictable connections to distant neurons or other target cells. Failure in connecting to the correct targets may result in severe defects. The number of known neurodevelopmental disorders related to aberrant axon connectivity in humans as a consequence of defective axon guidance is increasing rapidly. Almost everything we currently know about how axons are guided to their destination is based on biochemical signaling between neurons and their environment. Research on model organisms has provided tremendous advances in our understanding of the molecules (e.g., chemical guidance cues and their receptors) that govern axonal growth and guidance. However, axonal growth involves motion, and motion must involve forces. In spite of this, very little is known about the mechanical interactions of neurons with their environment. In the proposed project, we will, for the first time, investigate what mechanical signals axons encounter in the developing embryonic brain, how these signals change, and how they contribute to controlling axon growth. To achieve this goal, we will develop an experimental time-lapse technique, which is based on atomic force microscopy, to measure the dynamic mechanical properties of living Xenopus brain tissue, and simultaneously observe growing neuronal axons within the investigated area. Specific labeling of the neurons by the genetic insertion of a fluorescent protein will help distinguishing them from the rest of the tissue. This way, we will learn where and when in the brain stiffness gradients are found, how local tissue stiffness changes, and if neuronal axons show a preference for a certain range of tissue stiffness as suggested by preliminary experiments of our laboratory. Once we know the mechanical properties of the brain at each developmental stage, we will illuminate which cellular and extracellular structures are responsible for the measured properties. Finally, we will locally and globally interfere with tissue stiffness using pharmacological, genetic and physical approaches, and observe the effect of our treatment on axonal pathfinding. Thus, this project will illuminate a possible involvement of developmental tissue mechanics in neuronal growth and guidance. It has hence a great potential to lay the foundation for a new branch of developmental neurobiology, and it could ultimately greatly contribute to understanding and preventing and / or treating neurodevelopmental and also psychiatric disorders. Ultimately, knowledge gained in this project could also tremendously contribute to facilitate regeneration of damaged neurons after spinal cord injuries, which are currently incurable, and which have so far being investigated mostly from a biochemical point of view.